Philips Atlas of the Universe

objects including open and globular clusters, gaseous

nebulae and galaxies. Messier’s catalogue contained

slightly over a hundred objects. His numbers are still

used; thus the Andromeda Spiral is M31, the Orion

Nebula M42, the Crab Nebula M1, and so on.

Meteor:Cometary debris; a small particle which

enters the Earth’s upper atmosphere and burns away,

producing the effect known as a shooting star.

Meteorite: A larger body, which is able to reach

ground-level without being destroyed. There is a

fundamental difference between meteorites and

meteors; a meteorite seems to be more nearly related

to an asteroidor minor planet. Meteorites may be

stony (aerolites), iron (siderites) or of intermediate type.

In a few cases meteorites have produced craters; the

most famous example is the large crater in Arizona,

which is almost 1.5 kilometres in diameter and was

formed in prehistoric times.

Meteoroids: The collective term for meteoritic bodies.

It was once thought that they would present a serious

hazard to spacecraft travelling outside the Earth’s

atmosphere, but it now seems that the danger is very

much less than was feared, even though it cannot be

regarded as entirely negligible.

Micrometeorite: An extremely small particle, less

than 0.01016 centimetres in diameter, moving around

the Sun. When a micrometeorite enters the Earth’s

atmosphere, it cannot produce a shooting-star effect,

as its mass is too slight. Since 1957, micrometeorites

have been closely studied from space probes and

artificial satellites.

Micron: A unit of length equal to one thousandth of a

millimetre. There are 10,000 Ångströmsto one micron.

The usual symbol is Ì.

Midnight Sun: The Sun seen above the horizon at

midnight. This can occur for some part of the year

anywhere inside the Arctic and Antarctic Circles.

Milky Way: The luminous band stretching across

the night sky. It is due to a line-of-sight effect; when

we look along the main plane of the Galaxy (that is,

directly towards or away from the galactic centre) we

see many stars in roughly the same direction. Despite

appearances, the stars in the Milky Way are not closely

crowded together. The term used to be applied to the

Galaxy itself, but is now restricted to the appearance

as seen in the night sky.

Millibar: The unit which is used as a measure of

atmospheric pressure. It is equal to 1000 dynes per

square centimetre. The standard atmospheric pressure

is 1013.25 millibars (75.97 centimetres of mercury).

Minor planets: See asteroids.

Molecule: A stable association of atoms; a group of

atoms linked together. For example, a water molecule

(H 2 0) is made up of two hydrogen atoms and one atom

of oxygen.

Month:(1) Calendar month: the month in everyday

use. (2) Anomalistic month: the time taken for the Moon

to travel from one perigeeto the next. (3) Sidereal

month: the time taken for the Moon to complete

one journey around the barycentre,with reference

to the stars.

Multiple star: A star made up of more than two

components physically associated, which orbit their

mutual centre of gravity.

N

Nadir:The point on the celestial sphere immediately

below the observer. It is directly opposite to the

overhead point or zenith.

Nebula: A mass of tenuous gas in space together

with what is loosely termed ‘dust’. If there are stars in

or very near the nebula, the gas and dust will become

visible, either because of straightforward reflection

or because the stellar radiation excites the material to

self-luminosity. If there are no suitable stars, the nebula

will remain dark, and will betray its presence only

because it will blot out the light of stars lying beyond

it. Nebulae are regarded as regions in which fresh stars

are being formed out of the interstellar material.

Neutrino: A fundamental particle which has no mass

and no electric charge – which makes them extremely

difficult to detect.

Neutron: A fundamental particle whose mass is equal

to that of a proton, but which has no electric charge.

Neutrons exist in the nuclei of all atoms apart from that

of hydrogen.

Neutron star: A star made up principally or

completely of neutrons, so that it will be of low

luminosity but almost incredibly high density.

Theoretically, a neutron star should represent the

final stage in a star’s career. It is now thought probable

that the remarkable radio sources known as pulsars

are in fact neutron stars.

Newtonian reflector: The common form of

astronomical reflector. Incoming light is collected

by a mirror, and directed on to a smaller flat mirror

placed at 45 degrees. The light is then sent to the side

of the tube, where it is brought to a focus and the

eyepiece is placed. Most small and many large

reflectors are of Newtonian type.

Noctilucent clouds: Rare, strange clouds in the

ionosphere, best seen at night when they continue

to catch the rays of the Sun, after it has set. They lie

at altitudes of greater than 80 kilometres, and are

noticeably different from normal clouds. It is possible

that they are produced by meteoritic dust in the upper

atmosphere.

Nodes:The points at which the orbit of a planet,

a comet or the Moon cuts the plane of the ecliptic,

either as the body is moving from south to north

(ascending node) or from north to south (descending

node). The line joining these two points is known as

the line of nodes.

Nova: A star which undergoes a sudden outburst,

flaring up to many times its normal brilliancy for

a while before fading back to obscurity. A nova is a

binary system in which one component is a white

dwarf; it is the white dwarf which is responsible for

the outbursts.

Nutation:A slight, slow ‘nodding’ of the Earth’s

axis, due to the fact that the Moon is sometimes above

and sometimes below the ecliptic, and therefore does

not always pull on the Earth’s equatorial bulge in

the same direction as the Sun. The result is that the

position of the celestial pole seems to ‘nod’ by about

9 seconds of arc to either side of its mean position

with a period of 18 years 220 days. Nutation is

superimposed on the more regular shift of the

celestial pole caused by precession.

O

Object-glass (objective): The main lens of a

refracting telescope (see refractor).

Obliquity of the ecliptic: The angle between the

ecliptic and the celestial equator. Its value is 23 degrees

26 minutes 54 seconds. It may also be defined as

the angle by which the Earth’s axis is tilted from the

perpendicular to the orbital plane.

Occultation: The covering up of one celestial body

by another. Thus the Moon may pass in front of a

star or (occasionally) a planet; a planet may occult

a star; and there have been cases when one planet

has occulted another – for instance, Venus occulted

Mars in 1590. Strictly speaking, solar eclipses are

occultations of the Sun by the Moon.

Opposition: The position of a planet when it is exactly

opposite the Sun in the sky, and so lies due south at

midnight. At opposition, the Sun, the Earth and the

planet are approximately aligned, with the Earth in

the mid position. Obviously, theinferior planets

(Mercury and Venus) can never come to opposition.

Orbit:The path of an artificial or natural celestial

body. See also transfer orbit.

Ozone:Triatomic oxygen (0 3 ). The ozone layer in the

Earth’s upper atmosphere absorbs many of the lethal

short-wavelength radiations coming from space. Were

there no ozone layer, it is unlikely that life on Earth

could ever have developed.

P

Parallax, trigonometrical: The apparent shift of

a body when observed from two different directions.

The separation of the two observing sites is called

the baseline. The Earth’s orbit provides a baseline

300 million kilometres long (since the radius of the

orbit is 150 million kilometres); therefore, a nearby

star observed at a six-monthly interval will show a

definite parallax shift relative to the more distant stars.

It was in this way that Bessel, in 1838, made the first

measurement of the distance of a star (61 Cygni). The

method is useful out to about 300 light-years, beyond

which the parallax shifts become too small to detect.

Parsec:The distance at which a star would show

a parallax of one second of arc. It is equal to 3.26

light-years, 206,265 astronomical units, or 30.8 million

million million kilometres. (Apart from the Sun, no star

lies within one parsec of us.)

GLOSSARY

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